A hearing aid system of one general type to which the present invention relates is disclosed in U.S. Pat. No. 4,982,434--Lenhardt et al. In the referenced patent, there is disclosed a hearing aid system which utilizes such shifting of signals from the audio frequency range to the ultrasonic frequency range (referred to as "supersonic" frequency range in the patent), conversion of the shifted ultrasonic signal to a vibratory signal, and physical application of the vibratory signal to the human body for communication with the human sensory system. In one embodiment of the invention as disclosed in the referenced patent, an audio frequency signal is amplitude modulated onto an ultrasonic carrier signal for conversion to a vibratory signal and application to the body. In that embodiment, amplitude modulation is carried out by utilizing the analog audio signal as the modulating signal to modulate an analog ultrasonic carrier signal. In such a modulation system, an amplitude modulated signal with double (upper and lower) sidebands is derived.
The referenced system has provided excellent results in permitting the severely hearing impaired and even otherwise totally deaf persons to sense and understand audio frequency communications. It is an object of the present invention to provide even further improvements in systems of the aforementioned type and to provide as well improved apparatus and methods for shifting the frequency of audio frequency signals from one frequency band to another to improve the hearing response of hearing impaired persons who have a more acute hearing response in a frequency range which differs from the normal response in the audio frequency range. It is also an object of the present invention to provide improved digital apparatus and methods for single sideband amplitude modulation for hearing aid applications as well as other applications. Such digital frequency shifting may be either up or down in frequency from the normal audio frequency range and may be into the ultrasonic range as well into other frequency ranges depending upon the hearing response characteristics of the subject.